If I only eat 700 calories a day and excersize how much weight will I lose?

Okay so I weigh 143 and am 5'5. I eat like 5 times a day but it's usually low calories things. Like Fat free yogurt, bran flakes,fruit. By the end of the day it adds up to about 700 calories. I speed walk for about 20 minutes a day. How fast will I lose weight?